In general, Koehler illumination optical systems are widely used as the illumination optical system for microscopes because it makes the brightness of the field of view uniform FIG. 7 shows the basic construction in the case where a Koehler illumination optical system is used with a microscope to illuminate an object using light that is incident substantially normal to a support surface 9 which supports an object of interest. In FIG. 7, a light source 1 emits illumination light which is gathered by a collector lens 2 and passes this light via a first illumination lens 3 an aperture stop 4 a field stop 5 a second illumination lens 6, a dichroic mirror 7 (which may instead be a beam splitter), and an object lens 8 to a sample surface 9. The collector lens 2 the first illumination lens 3 the aperture stop 4 the field stop 5 and the second illumination lens 6 form the illumination lens system.
As compared to the viewing area obtainable by using one's eyes to view a magnified image of a sample by looking into a microscope, only a more narrow viewing area in the vicinity of the center of the field of view can be observed using, for example, a CCD to capture the image and to output the image data to a display, such as a T.V. monitor. For an illumination optical system for a microscope that is more suitable for T.V. viewing, what is termed ‘critical illumination’ is often used. This type of illumination optical system projects an image of a light source onto a center portion of a sample surface to be viewed, brightly illuminating the center portion of the field of view.
As shown in FIG. 8A, a magnified image of a sample positioned at sample surface 9 may be viewed in reflected light using the objective lens 8 and an imaging lens (tube lens). In this case, the reflecting illumination optical system is formed of the light source 1, the illumination lens system (composed of the components 2-6, discussed previously), the dichroic mirror 7, and the objective lens 8. As shown in FIG. 8B, a magnified image of a sample positioned at a sample surface 9 may be viewed in transmitted light using an objective lens and an imaging lens. In this case, the transmitting illumination optical system is formed of the light source 1, the illumination lens system, the dichroic mirror 7, and a condenser lens. Because the components of the illumination lens system are the same as in the Koehler illumination optical system shown in FIG. 7, the components themselves will not be separately discussed with regard to the illumination optical systems shown in FIGS. 8A and 8B. In FIGS. 8A and 8B, the illumination light that is incident onto the sample surface 9 that supports a sample is not collimated, as in the case of Koehler illumination illustrated in FIG. 7. Instead, a focused image of the light source is projected onto the sample surface 9. This is termed critical illumination.
The light source used for a microscope is generally a halogen lamp or an arc-discharge lamp, both of which have a small region that emits a high intensity light beam. For example, this region for an arc-discharge lamp typically measures about 0.6 mm in diameter. If an arc-discharge lamp is used in Koehler illumination, the projected image of the lamp is generally too small to fill the pupil of the objective lens of the microscope, and thus the brightness of the field of view decreases. Therefore, there has been a problem using a Koehler illumination optical system as shown in FIG. 7 in that both the required illumination field (i.e. the area of illumination) and the required brightness of illumination cannot be simultaneously provided. However, using a critical-type illumination optical system as illustrated in FIG. 8A for viewing the sample using reflected light, or as illustrated in FIG. 8B for viewing the sample using transmitted light, also has problems in that only the center portion of the field of view can be brightly illuminated. Thus, neither type of illumination optical system is fully satisfactory in terms of practical usage.
An illumination optical system for a microscope is provided with a light source, an illumination lens system which gathers light from the light source and directs the light along a light path, and an object optical system which converges the light beams from the illumination lens system so as to illuminate a sample for observation with a microscope using either transmitted or reflected light. The object optical system is formed of the objective lens 8, as shown in FIG. 8A, in the case of using reflected light to view a sample. In the case of using transmitted light to view a sample, the object optical system is instead formed of a condenser lens, as illustrated in FIG. 8B.
The microscope optical system includes an objective lens and an imaging lens that, together, form a magnified image of the sample. In the case of using reflected light to view a sample, the objective lens 8 (FIG. 8A), serves a dual role. It not only serves as part of the illumination optical system to converge the light from the illumination lens system that is to illuminate the sample, it also serves as the objective lens of the microscope to gather the light reflected by the sample and to direct the light to the imaging lens of the microscope.